This invention relates to a method of measurement of the concentration of a substance contained in a gas and to devices for carrying out said method. The invention finds an application especially in the measurement of the concentration of water vapor contained in a gas and in particular in the atmosphere. It can also apply to the detection of contaminants in the atmophere.
When it is desired to measure the concentration of a substance contained in a gas, either the chemical or the physical properties of this substance are utilized in order to distinguish it from other substances which are present in the gas, a quantitative analysis of said substance being accordingly performed by means of these properties. It is thus a known practice to make use of optical methods based on measurement of the absorption exhibited by the substance to be determined at particular wavelengths or on the measurement of the luminescence which it exhibits under the action of various excitations.
All of these known methods are distinguished by the fact that the analysis is performed directly on the substance whose concentration is to be ascertained. In some instances, however, the physical or chemical characteristics of the substance having a concentration which it is desired to determine differ only to a slight extent from those of other substances which are present in the gas. This is the case, for example, of water vapor since the absorption spectrum exhibited by this latter cannot readily be differentiated from the spectra of other gases which are usually present in the atmosphere together with the water vapor. In this case the known methods are difficult to apply and are lacking in accuracy.